1. Field of the Invention
The present invention relates generally to a soft handover method in an asynchronous Code Division Multiple Access (CDMA) mobile communication system, and in particular, to a soft combining apparatus and method for soft handover in Multimedia Broadcast/Multicast Service (MBMS).
2. Description of the Related Art
Due to recent developments occurring in the tele-communication industry, CDMA mobile communication systems are required to provide a multicasting multimedia communication service for transmitting large volumes of data, such as packet data and circuit data, as well as providing voice service.
In order to support multicasting multimedia communication, a service known as Broadcast/Multicast Service is available for providing a service from one data source to a plurality of user equipments (UEs). Broadcast/Multicast Service can be classified into Cell Broadcast Service (CBS), supporting a message-based service, and Multimedia Broadcast/Multicast Service (MBMS), supporting a multimedia service for transmission of real-time image and voice data, still images, and text data.
The CBS service provides a plurality of messages to all UEs located in a particular service area. The specific service area where the CBS service is provided can become an area providing CBS service when the CBS service is provided within one cell.
The MBMS service, which simultaneously provides voice data and image data, requires a large amount of transmission resources. The MBMS service is provided through a broadcasting channel with a view towards the possibility that a plurality of services can be simultaneously performed within one cell. In particular, the MBMS service requires more radio resources when compared to the CBS service. Therefore, messages in an MBMS service are actually broadcasted only in a service area where a UE can receive the MBMS service.
Since a UE typically does not stay in one cell area, but moves from one cell area to another cell area, an the existing voice service and data service of that UE can be continued through soft handover. During data transmission/reception, handover of service can be roughly classified into handover in a CELL_DCH state and handover in a CELL_FACH state.
Handover in a CELL_DCH state refers to handover performed in a state where a dedicated channel (DCH) is set up. In the handover procedure in a CELL_DCH state, a UE measures a common pilot channel signal (CPICH) and reports the measurement result to a serving radio network controller (SRNC). The SRNC determines whether to perform handover based on the report from the UE, and sends a handover command to the UE according to the determination result. Therefore, it is possible to soft-combine the report from the UE with the handover command from the SRNC.
Handover in a CELL_FACH state refers to handover performed in a state where a forward access channel (FACH) is set up. In the handover procedure in a CELL_FACH state, a UE determines a best cell by itself by measuring CPICH thereby to perform cell update. In this case, it is not possible to perform the soft-combining, because the UE makes no report to the SRNC. Therefore, the MBMS service, which should consider the handover in a CELL_FACH state as it is provided through a broadcasting channel, faces a soft combining problem.